Various methods for the preparation of copolyester-carbonates are described in U.S. Pat. Nos. 3,030,331, 3,169,121, and, 3,207,814, all of which are hereby incorporated by reference. A process for obtaining aromatic polycarbonates wherein the pH is maintained between 8.0 and 10.2 is disclosed in U.S. Pat. No. 3,989,672 which is also incorporated herein by reference. In addition, copending application Ser. No. 755,352, assigned to the same assignee as this case, discloses a method for preparing polyester carbonates by an interfacial polymerization process wherein the copolymers obtained contain both carboxylate and carbonate groups. Copending application Ser. No. 844,285, also assigned to the same assignee as this case, discloses a method for preparing copolyester-carbonates from a mixed polyanhydride ester of a dicarboxylic acid and a halogen carbonic acid ester of a polyhydroxy compound, and copending application Ser. No. 33,393, filed Apr. 26, 1979 and assigned to the same assignee as this case discloses a process for obtaining copolyester-carbonates employing diacid chlorides and following a pH profile during the process.
While these prior art methods are useful, they are not completely satisfactory as they either involve complex chemical reactions and costly equipment, or require expensive starting materials, or are time consuming and, therefore, uneconomical to manufacture.